Ion exchange technologies are widely used for water and waste treatment in hydrometallurgy, biochemistry, medicine, environmental protection and many other industries. It is well known that their efficiency depends on many factors, the main among them being the selectivity of the exchanger in use. Inorganic ion exchangers and adsorbents, due to such properties as chemical and thermal stability, resistance to oxidation and unique selectivity to certain ions, have definite advantages in comparison with well known and traditionally used organic resins. It was found that inorganic ion exchangers are able to operate in extreme conditions (high temperature or strong radiation fields, in the presence of organic solvents and/or oxidants and in a great excess of competitive ions), where organic resins fail to work efficiently. Among the known inorganic adsorbents zirconium phosphates (ZrP) have been studied in detail. Zirconium phosphates can be amorphous or crystalline and have a general formula ZrO2.nP2O5.mH2O.(xMO), where n=0-2.5, m>0, x=0-5, M=metal ion.
Conventional methods of preparing amorphous zirconium phosphates include reaction between aqueous solutions of a zirconium salt and a phosphorus containing reagent, such as phosphoric acid or its salts, with an instantaneous formation of a gel precipitate. Thereafter the precipitate is filtered, washed and/or subjected to any additional treatments and dried. The final product after drying is a fine powder or granules with irregular form. The large amount of experimental work performed in this field is summarized in several review books (Amphlett, C. B. Inorganic Ion Exchangers. Elsevier, New York (1964); Clearfield A., Ed. Inorganic Ion Exchange materials, C.R.C. Press USA, 1982; Vesely, V. and Pekarek, V. Synthetic inorganic ion exchangers, Talanta. 19, 219 (1972) and patents, including U.S. Pat. Nos. 3,056,647; 3,485,763; 4,025,608; 4,381,289, and 2,349,243, all of which are incorporated herein by reference. Depending on the experimental conditions, such as pH, temperature, duration, etc., and composition of the reaction mixture, the P/Zr ratio in the final product can vary in a broad range from ˜0 up to 2.0. Amorphous products typically contain HPO4, H2PO4, PO4 and, in some cases, Zr—OH groups. The presence of phosphorus-containing functional groups such as HPO4 and H2PO4 provides cation exchanger properties to zirconium phosphates. As discussed in S. Ahrland, et al, J. Inorg. Nucl. Chem., 32, 2069 (1970), some amorphous zirconium phosphates show affinity towards transition metals. However, amorphous zirconium phosphates synthesized via the precipitation route have several drawbacks, including:                strong dependency between ion exchange performance and moisture content, which suggests loss of capacity and deterioration of kinetics of sorption with the loss of water during storage or under drying;        low thermal stability;        poor mechanical and hydrodynamic properties of sorbents (powders, granules of irregular form), preventing use for column type applications.As discussed in French Pat. No. 1,317,359 (1963); U.S. Pat. No. 4,806,517 and C. Y. Yang, Separ. Sci. & Techn., 18, 91 (1983), amorphous zirconium phosphates in powdered form can be granulated with the use of organic or inorganic binders. This approach allows the production of mechanically strong ion exchangers in the form of beads or extrudates of desired shape suitable for column applications. However, use of binders affects the total ion exchange capacity, the kinetics of adsorption and limits certain applications due to solubility of the binder and possibility of additional contamination of the product.        
Granulated amorphous zirconium phosphates without the use of binders can be prepared via sol-gel or gel routes. Sol-gel granulation processes based on the oil-drop principle include conversion of ZrO2 sol into hydrous zirconium oxide gel (spherical granules with a particle size of 0.1-3 mm) in organic water immiscible media followed by conversion into zirconium phosphate by treatment of the ZrO2 gel with a phosphoric acid or phosphoric acid salt (R. Caletka, M. Tympl, J. Radioanal. Chem., 30: 155 (1976). The gel method, also based on the oil-drop principle, includes reaction between aqueous solutions of zirconium salt and phosphoric acid (or its salt) in the presence of Zr-complexing reagent such as H2O2, polyatomic alcohols and organic oxyacids, which allows a direct formation of zirconium phosphate gel (Amphlett, C. B. Inorganic Ion Exchangers. Elsevier, New York (1964); V. V. Strelko, Chemistry Role in the Environmental Protection, p. 179, Naukova Dumka, Kiev (1982)). Spherically granulated zirconium phosphate sorbents prepared via sol-gel and gel routes have high crush strength and good attrition resistance. However, they still have drawbacks of a strong dependency between ion exchange performance and moisture content, as well as low thermal stability.
Another method of making granulated zirconium phosphate is described in U.S. Pat. No. 4,025,608. According to this method zirconium phosphate is made by the reaction of a zirconium salt, having a predetermined particle size of 30-40 microns, with phosphoric acid or a phosphate in a liquid medium. Amorphous zirconium phosphate made according to this method also has drawbacks of a strong dependency between ion exchange performance and moisture content, as well as low thermal stability.
Crystalline zirconium phosphates can be prepared by treatment of amorphous zirconium phosphates in the presence of excess of H3PO4 at elevated temperature for long periods of time (A. Clearfield, J. A. Stynes, J. Inorg. Nucl. Chem., v.26, 117, (1964), U.S. Pat. Nos. 3,130,147 and 4,695,642); by slow decomposition of fluoro-zirconium complexes in the presence of H3PO4 (G. Alberti et al, J. Inorg. Nucl. Chem., v.30, 317, (1968)); by reaction between aqueous solutions of a zirconium salt and a phosphorus containing reagent (phosphoric acid or its salts) under hydrothermal conditions (S. Komarneni, Int. J. High Tech. Ceram., 4, 31, (1988); M. K. Dongare et al, Mat. Res. Bull. v.27, 637-645, (1992)); and also via solid state reactions between ZrO2 or Zr salts and salts of phosphoric acid (J. M. Winand et al, J. Solid State Chem., 107, 356 (1993); V. A. Sadykov et al, Kinetics and Catal., 42, 344, (2001)). Depending on the experimental conditions, composition of the reaction, presence of templates, mixtures of different crystalline modifications of zirconium phosphate, both layered and framework, can be prepared. Among them are hydrated products like α-Zr(HPO4)2 H2O (A. Clearfield, J. A. Stynes, J. Inorg. Nucl. Chem., v.26, 117, 1964), γ-Zr(H2PO4)(PO4) 2H2O (A. Clearfield et al, J. Inorg. Nucl. Chem., v.30, 2249, 1968), τ-Zr(HPO4)2 H2O (A. M. K. Andersen et al, Inorg. Chem., v.37, 876-881, 1998), ψ-Zr2O3(HPO4) nH2O (A. Clearfield et al, Inorg. Chem. Comm., 1, 208 (1998), HZr2(PO4) (S. Feng, M. Greenblatt, Chem. Mater., v.4, 1257, 1992), or non hydrated materials like MZr2(PO4)3, MZr5(PO4)7 (M. K. Dongare et al, Mat. Res. Bull. v.27, 637-645, 1992), M5 (J. P. Boilot et al, J. Solid State Chem., 50, 91 (1983), ZrP2O7, Zr(OH)PO4 (N. G. Chernorukov et al, J. Inorg. Chem., v.28, 934 (1984), and so on.
Some crystalline zirconium phosphates contain exchangeable ions (H+ or metal cations) and show ion exchange properties. Selectivity of crystalline materials strongly depends on the type of crystal structure and, in some cases, is much higher than that of amorphous compounds. Another advantage of crystalline materials is that they are less susceptible to moisture content than amorphous sorbents and, as result, are more thermally stable. However, disadvantages of crystalline ion exchangers include poor adsorption kinetics and their powdered form, which prevents their use in column applications.